Chimpanzees Sleep in Trees to Escape the Humidity

Making nests in trees keeps chimps comfortable and safe from nighttime predators

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Sometimes sleeping on the ground is cooler and more comfortable for chimpanzees. Image courtesy of Flickr user purpleairplane

Last month, I wrote about how chimpanzee sleeping habits help anthropologists study the sleeping behavior of early hominids. Chimps typically build nests in trees when it’s time to go to sleep. Having an arboreal bed is likely a way to stay safe from prowling nighttime predators. Chimpanzees living in the Nimba Mountains of Guinea often sleep on the ground, probably because no chimp-eating animals live there. Millions of years ago, early hominids may have acted similarly.

But the study I reported on in April couldn’t explain why chimps preferred to sleep on the ground when it was an option. Recent work provides some answers.

David Samson and Kevin Hunt of Indiana University studied chimpanzee nesting in Uganda. They thought that differences in microclimate between the canopy and the ground might explain where the animals prefer to sleep. The pair set up portable weather monitors in trees and on the ground near nests from August 2010 to January 2011.

The ground is a cooler, less windy place to sleep, Samson and Hunt reported in the American Journal of Primatology. Chimpanzees sleeping in terrestrial nests probably spend less time trying to keep their beds stable in the face of unexpected gusts and therefore probably sleep more soundly throughout the night. Furthermore, based on estimates of temperature, wind speed, humidity and chimpanzee body mass, the researchers say the animals sleeping on the ground stay in “energy balance” while those sleeping in trees experience more thermal stress. In other words, sleeping on the ground is a more comfortable option. Still, despite the benefits of ground-sleeping, most chimpanzees in the Uganda study area have to sleep in trees because of the area’s lions and leopards.

The Nimba Mountain chimpanzees don’t have to worry about predators, so comfortable conditions on the ground could explain why ground-nesting is so common there. That’s something that still need to be tested, however. What’s happening in Uganda may not apply to the Nimba Mountains because the two regions have different habitats and climates; the Nimba Mountains are wetter.

And at least during part of the year, that wetness seems to deter ground-sleeping, suggests a recent study in the International Journal of Primatology. Over the course of 28 months in 2003 to 2008, Kathelijne Koops of the University of Cambridge in England and colleagues discovered tree-sleeping was most common during the wet season in the Nimba Mountains. During this time, chimps preferred sleeping in higher altitudes (more than 3,000 feet) and higher up in trees (almost 38 feet high). Koops and colleagues thought this could be a way to avoid disease-carrying mosquitoes, but the bugs were equally common throughout the year. Instead, it turns out chimpanzees make arboreal nests during the wettest time of the year to avoid humidity, which is higher near the ground and in lower elevations.

These recent studies reveal chimpanzee nesting is more complicated than just being a predator-deterrence strategy. Anthropologists should keep that in mind when they study early hominid behavior, too.

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